Creating a precast structure that will retain its durability and aesthetic appeal over time remains a key concern of owners and designers. Although first costs always are among owners’ top priorities, lifecycle and long-term maintenance expenses can eliminate any initial savings. Recently, two detailed studies of precast components sponsored by the Precast/Prestressed Concrete Institute (PCI) were released that can help owners and designers balance these needs.

1. Studies Offer Proof
DURABILITY
Of Precast’s Durability
Survey of parking garages offers tips on design and maintenance, while
chloride-permeability tests show existing standards are confusing
C
reating a precast structure that Northbrook, Ill. The one-year Durability in parking structures can be
will retain its durability and experimental study was designed to enhanced with a lower water/cement ratio
and a long-term maintenance plan.
aesthetic appeal over time answer questions relating to chloride Photo: Hedrich-Blessing
remains a key concern of owners and permeability, water-absorption rates,
designers. Although first costs always and other durability considerations.
are among owners’ top priorities, life- Among its findings was that in many
cycle and long-term maintenance cases, accepted tests for determining the
expenses can eliminate any initial durability of concrete needed for a
savings. Recently, two detailed studies project may be overstated, costing
of precast components sponsored by the owners money by making them pay for
Precast/Prestressed Concrete Institute methods and materials that aren’t
(PCI) were released that can help necessary.
owners and designers balance these
needs. Parking Maintenance
“Durability of Precast Prestressed CEG’s study of parking garages
ASCENT, SUMMER 1996
Concrete Structures” reports on the found that “the overall performance of
long-term durability of parking precast prestressed concrete parking
structures nationwide. The study was structures can be rated as very good,
conducted by The Consulting with the general performance rated as
Engineers Group Inc. (CEG) of San 84 percent out of 100 percent.” The
Antonio. The report summarizes a primary areas of concern if problems
28 survey undertaken by CEG of 62 were to arise, it notes, are derived from
parking structures. The firm evaluated field-placed toppings, excessive leaking,
various performance characteristics and poor drainage-system design, and
inspected several sites to determine what cracking in precast spandrels. This latter
methods and design criteria work best rating area was due primarily to hairline
in various situations. cracks, the study says. Even so, it notes,
In general, the study found that pre- “The performance of the precast
cast parking garages perform extremely products was excellent,” and these
well over time and those that are well- problems seldom arose.
maintained and regularly inspected However, it found that maintenance
perform best. It also offers a host of efforts average just 5.76 on the 10 scale.
suggestions on how to achieve these This represents a major concern, as lack
ends through design, construction, and of maintenance often lies at the heart of
on-going inspection plans. any problems that do arise with precast
The second report, “Durability garages, it notes.
Aspects of Precast Concrete, Parts I and The examined structures ranged in
II, Historical Review and Chloride age from two to 24 years and were
Permeability Study,” was conducted by located in 17 states across the country.
the engineering firm of Wiss, Janney, The dimensions were representative of
Elstner Associates Inc. (WJE) of typical structures, the study says, with a

2. minimum width of 105 feet, a particularly when it freezes. crickets that direct water to the drains.
maximum length of 1,200 feet, and up Good drainage includes five key This, in fact, was achieved for most of
to 12 levels of parking. About 65 components, the study reports: primary the garages studied.
percent of the structures were located in span floor slope, cross slope, drain Drains located as close to the column
regions where salt was applied, with 69 grid lines as possible minimize local
percent in regions where freeze-thaw ponding, while drains measuring at least
conditions were a consideration. Some ‘Poor drainage was 11 inches in diameter perform the best,
41 percent of the structures also fea-
tured warped double tees, which remains
found to be one of the the study reports. Trench drains are
used at the bottom of the roof-level
a key focus for design considerations. principle culprits in the ramp. They tend to be spaced so they
Concrete strength averaged 5,878 psi don’t interrupt the floor-framing sys-
and ranged from 5,000 to 7,000 psi. deterioration of tem, rather than being used continu-
These key areas were spotlighted. ously.
parking structures.’
Proper Drainage A Must Sealants Make A Difference
“Poor drainage was found to be one location, drain size, and joint sealing. Sealants play a critical role in
of the principle culprits in the deteriora- Excessive slopes can lead to warping maintenance programs, the study
tion of [the studied] parking problems, minimized headroom, or found. “The structures that performed
structures,” the study reports. In the few safety issues, the study says. The goal the best were sealed at all circumstances
cases where it occurred, the study says, should be to create a minimum slope where precast members come together,”
poor drainage allows water to collect on that still drains water efficiently. The it says. This includes joints connecting
the deck’s surface, creating ponds that survey found that in most cases, the flange to flange, tee to beam and
allow road salt to concentrate and attack desired slope ran between 1.5 percent spandrel, tee and beam to wall panels,
the concrete. This in turn lets chloride and two percent for the primary long- and all those around columns. If these
ions penetrate the concrete. Ponding span slope and 0.5 percent to one were properly sealed, the study says,
also can create a safety problem, percent for the beam lines that create across the board “the deck had very few
ASCENT, SUMMER 1996
29

3. if any leaks.” Urethane was the most flange cracks if a field-placed topping is be ones that minimize field-placed
typical sealant used, the study adds. to be applied.” If the system is pre- concretes which typically have high w/c
“Properly installed urethane appears to topped, however, even small cracks ratios,” the report summarizes. “The
have an effective life of seven to 10 should be sealed to ensure water and pretopped double tee system would
years, with touch-up maintenance when water-borne chlorides do not attack the produce the most durable concrete at
and where required.” underlying system. the riding surface as well as throughout
Joints should be prepared properly Any cracks in pretopped systems, the structure. If field-placed topping is
prior to sealing them, the study stresses. should they arise, as well as larger cracks employed, then a w/c ratio of no more
In topped systems the topping should in field-topped decks, typically are than 0.45 is recommended.” In fact,
be tooled (not sawn) to a depth of 1/2 sealed by a surface application of a clear, PCI recommends that field-placed
inch and all scales or latence removed low-viscosity epoxy, the report adds. topping should offer a w/c ratio as low
by grinding. Flange joints in pretopped Working cracks are best treated with a as possible, preferably 0.40 or lower.
systems should be ground or stoned to sealant such as urethane. It also says that Steel embeds often are left exposed to
remove sharp edges and loose material. cracks larger than 0.0010 inch should the elements in a precast structure, the
be examined by an experienced engi- study observes, and these usually consist
Flange Cracking neer. Silane sealants have also proven to of precast-member bearing plates and
Strains caused by shrinkage create be effective for small cracks (less than connection plates between precast
most of the cracking in double tee 0.0005 inch). members in welded connections. These
flanges that occurs, the study reports, Although no type appeared in the can be subjected to water from leaks or
emphasizing that such cracking is not surveyed garages with regularity, harsh climates and should be protected.
typical. These cracks most often develop spandrel cracking was reported to be the In regions where road salt isn’t used,
most prevalent type, the report says. only a shop paint rust preventative coat
Such cracking typically is observed as a is needed to ensure smooth operation, it
‘Flange cracking, unless diagonal torsion crack, particularly in L- reports. In regions where salt is used,
beam spandrels or cracks radiating from the embeds should be galvanized or
severe, seldom leads to the bearing or connections. To alleviate protected with zinc or epoxy paint. It
this, connections should be used that also points out that regular maintenance
poor performance of the allow spandrels to relieve temperature of the structure will provide additional
strains, the report suggests. “Bolted protection by ensuring caulks and
double tee.’ connections have proven to perform sealers are working properly.
quite well,” it notes. In addition, “Fiber-
during the production and curing reinforced bearing pads at least one- Shear Walls Predominate
cycles, and they build up in the flange half-inch thick also were observed to The predominant lateral-load system
regions between the stems. During the perform better than plain bearing pads.” used by the studied garages consisted of
curing process, the stem concrete is held a shear wall system. Some 89 percent of
rigidly by the steel forms. This restrains Admixtures Can Help the structures examined used this ap-
ASCENT, SUMMER 1996
the flange’s ability to shrink between “Certain admixtures are fairly univer- proach, while 10 percent used moment
the stems as it cures, occasionally sally employed by precast concrete frames and one percent used only stair
producing a crack in the flange adjacent producers to greatly enhance the dura- and elevator shafts. However, examina-
to the inside corner of the juncture bility of precast concrete products,” the tion of the systems showed there is no
between the stem and the flange. study reports. The two most common need to isolate stair and elevator shafts
“These cracks can be minimized by are air-entraining agents used in away from the structure, the study
30 proper curing techniques that retain freeze/thaw regions and super plasti- found. “If appropriate connections are
moisture,” the report says. cizers that reduce the amount of water
Cracking also can occur during the needed to produce a workable concrete
stripping process, it notes, when one mix, thereby lowering the water/cement
end of the stem is elevated higher, (w/c) ratio.
putting more stress on the other end. It The report refers to the tests and
also can result from excessive wracking experiments run in the WJE study,
of the tee as it is transported to the adding that its own survey bears out the
jobsite. Although none of these occur findings that reducing the w/c ratio to
often, they are the key areas to focus on within the range of 0.35 to 0.40 greatly
to prevent any cracking that may arise. reduces the ability of chloride ions to
Even then, the report team’s penetrate the concrete. “Concrete with
inspections showed that “flange crack- this w/c ratio has proven to be nearly
ing, unless severe, seldom leads to poor impervious, and the additions of other
performance of the double tee.” Small admixtures such as silica fume provide This diagram of a typical ductile bolted
cracks (those less than 0.004 inch in little substantial benefit.” It also notes connection shows the voids in the sleeve
width) “typically are not through cracks that 57 percent of respondents said they surrounding the bolt permit lateral movement
and seldom affect the performance of employed w/c ratios of this range in without impairing performance. In not one
case in the study where such a connection
the member,” the report says. “There- their projects already. was inspected was cracking at the connection
fore, nothing typically is done to such “The most durable structures would observed.

4. employed, such shafts can be included
in the lateral-load system.”
The most effective connection system
appeared to be one using bolted
connections between the spandrel and
the columns, with those connections
relieving thermal stresses. The
predominant connection type between
the shear walls themselves was a welded
connection in a recessed pocket.
Maximum Warping Analyzed
Double tees at times are warped
deliberately in a parking garage project This side view of a stress pattern in a warped double tee from CEG’s durability study shows the
when drainage plans provide different patterns of compression. The maximum tensile stresses occur in the top of the flange near the
slopes for the bearings at each end of unwarped stem, the survey found.
the double tee floor member. Although
this presents no problems in general and
can be accomplished routinely, the
report says, large warps may result in
some cracking in the double tee flange.
To measure the extent of this at
different amounts of warping, the
survey team ran a finite-element
analysis. The result showed that
maximum tensile stresses occur in the
top of the flange near the unwarped
stem. This follows classical bending-
stress distribution patterns, except the
top surface of the flange exhibits a stress
distribution similar to a concentrated
load-stress distribution. The results of
the finite-element analysis were This diagram shows the stress pattern that develops in a warped double tee that cracked. The
supported by an actual test of durable study found that the crack will propagate along the flange rather than through it, supporting
tees with various degrees of warping. the theory that most through-flange cracking is created by handling of the warped tee rather
than the actual warping process.
The study says that double tees can
ASCENT, SUMMER 1996
‘The overall Changes Suggested For AASHTO Standards
In its findings, the WJE study suggests several key changes to the existing
performance of precast AASHTO standards for curing and ponding tests.
“The current AASHTO moist- and heat- or steam-curing specifications [in
prestressed concrete Section 8.11] need to be revised,” it says. “There are numerous technical
parking structures can reasons for requiring moist-cured field concrete that will see chloride exposure 31
in its lifetime to have moist curing beyond the age when the concrete reaches
be rated as very good.’ 70 percent of the design strength.” The present provision allows these field
concretes to receive as little as one to three days of moist curing with existing
low w/c ratios and cements.
be warped safely to the amount of 3/4 It also suggests revisions to the T 259 Ponding Test. These include:
of an inch between stems and 1.5 inches
for a full tee width. In general, it adds, • A “more realistic” curing period, halving the current 14-day period.
“The stresses caused by a moderate • Use of a 15-percent chloride concentration solution for the ponding solution.
warp are generally not critical. The The existing three-percent NaCl solution represents sea water, not current
cracks resulting from a service warp are deicing salt solutions used in northern regions that are more concentrated and
generally longitudinal and do not harmful.
penetrate through the flange.” • Increased ponding periods from 90 days to 365. Since ponding tests require
Despite its focus on key areas of little supervision, this should add little impact to its cost, the study points out.
concern that arise with precast
construction, the report emphasizes that • Doubled chloride samples, from two to four, as two data points are “totally
this approach should not be construed insufficient to determine diffusion coefficients.”
to mean that precast garages offer many • Use of core rather than drilled samples, which can be contaminated and
difficulties. In fact, it stresses that just erroneous and provide inaccurate drilling depths.
the opposite was the case. “It should be

5. noted,” the report says in its summary, tested concrete sample was cured in a expresses the amount of electrical charge
“that all structures surveyed or inspected water tank, under wetted burlap or that passes through the concrete in a set
were in serviceable condition even when through heat-curing. Concretes incor- period of time and is measured in
over 25 years old and in severe porating admixtures of silica fume were coulombs. The ratings are supposed to
corrosion exposure and subjected to cured under wetted burlap only. allow designers and owners to specify
poor maintenance.” Even with its focus Three w/c ratios were produced, at the level of permeability they desire by
0.46, 0.37 and 0.32. These represented using the mixture’s coulomb rating as a
the two ends of the range typically delineator. However, the study found
‘The typical good found in precast products (0.37 to that ASTM’s existing coulomb ratings
0.32), plus one considerably higher in for concrete cured in each type of
performance of precast w/c, which is typical of 0.45 AASHTO- system do not correspond well to the
parking structures of all grade concrete. Samples using silica- actual permeability that can be seen in
fume additions of 5.0 percent and 7.5 practice.
kinds in all ranges of percent by weight of cement also were “Using coulomb ratings as an indica-
tested under wetted burlap, while tor of durability will be misleading and
exposure was apparent.’ standard precast concrete type mixes will add cost to a project,” says Donald
made with each of the three w/c ratios W. Pfeifer, project manager at WJE.
on finding where problems arise and were tested under each of the three This can be critical, because specifying
how they can be eliminated, “The types of curing. This resulted in 15 concrete to meet the set coulomb
typical good performance of precast different test samples. The results for ratings can be more expensive than
parking structures of all kinds in all each sample are shown in Table 1. using other concretes that are less costly.
ranges of exposure was apparent.” All 15 mixtures were tested according “The concrete w/c ratio is the
to standards set by the American Society dominant factor in reducing chloride
Chloride Ion Tests of Testing Materials (ASTM) to check permeability,” the study says. The
The ability to achieve high durability for permeable voids. The three heat- report found that the concrete with a
through various curing methods was cured mixtures of conventional concrete 0.37 w/c ratio had reduced its chloride
enhanced by the findings of the study were found to have the lowest volume ingress by about 80 percent over the
performed by WJE. It examined various of permeable voids. Adding silica fume concrete with a 0.46 w/c ratio. Further,
durability properties when cured in increased the number of voids by 100 the concrete with a 0.32 w/c ratio had
different ways and when created with percent for the five-percent mixture and reduced penetration of 95 percent. It
different w/c ratios. The study was 50 percent for the 7.5-percent version. pointed out that this evidence
conducted by WJE engineers for PCI The study also tested the coulomb confirmed a three-year corrosion study
and featured several formats. Each rating of each sample. The rating performed by the Federal Highway
ASCENT, SUMMER 1996
32
Chloride ingress, calculated diffusion coefficient and surface chloride and calculated times to corrosion for the 15 mixtures
Table 1, produced by WJE for its chloride-permeability study, summarizes the key findings of the tests. Among other things, it indicates that
lower water/cement ratio of any concrete sample correlated better with resisting chloride penetration than the AASHTO T277 coulomb ratings.

6. Complete Studies Available
“Durability of Precast Prestressed
Concrete Structures” was conducted
by Thomas J. D’Arcy, Walter J.
Korkosz and Larbi Sennour of the
Consulting Engineers Group Inc. of
San Antonio. Copies are available
from the company at 2455 N.E.
Loop 410, Suite 125, San Antonio
TX 78217; 210/637-0977.
The Chloride Permeability study
was conducted by Matthew Sherman,
David McDonald and Donald
Pfeifer of Wiss, Janney, Elstner
Associates Inc. of Northbrook, Ill.
Copies are available from PCI. (See
the in-depth story in the PCI
JOURNAL July/August ’96.)
Both reports can be obtained
from PCI, the sponsoring associa-
tion, at 175 W. Jackson Blvd., Suite
1859, Chicago IL 60604; 312/786-
This diagram from WJE’s chloride-permeability study shows that four different concretes
0300; e-mail, info@pci.org.
offering a wide range of coulomb ratings performed quite similarly during actual tests.
Administration in 1987. “The correla- The five-percent burlap-cured silica dramatically decreased the absorption
tion between long-term chloride fume mixture with a w/c ratio of 0.46 and volume of permeable voids in
had an estimated time-to-corrosion rate conventional concrete at all w/c levels.
essentially equal to that of 0.37 w/c “The reasons for and the implications of
‘Using coulomb ratings conventional heat-cured concrete, while these findings merit further research.”
the 7.5-percent burlap-cured mixture
as an indicator of with a w/c ratio of 0.37 corresponded to No Strength Loss
the heat-cured conventional concrete of The study also reports that “Proper
durability will be 0.32 w/c ratio. heat-curing created no significant
This shows two different approaches
misleading and will strength loss at 28 days at all three w/c
ASCENT, SUMMER 1996
to achieving the same durability, as levels.” This could be seen in the fact
add cost to a project.’ measured by a w/c ratio. However, the that 28-day strength of the material that
use of silica fume changes the overall received no supplemental moist-curing
characteristics, the study stresses. “The after heat-curing averaged 90 percent of
permeability and the coulomb test addition of silica fume to concrete the companion 28-day, water tank-
recommended in the ASTM C1202 test caused a significant increase in the water cured samples for all three w/c levels.
appears to be highly variable,” the study absorption and volume of permeable In addition, the study says, “The 33
says, “and requires individual correla- voids in moist-cured concrete at all w/c surface chloride concentrations for the
tion between these two types of tests for levels,” it says. Similarly, heat-curing heat-cured concretes are significantly
every concrete mixture.”
Heat-Cured Performs Well
Heat-cured concrete had the lowest
Changes Suggested For ASTM Standards
The WJE study team suggests five key changes to the ASTM C1202
water absorption and volume of
coulomb-testing standards:
permeable voids at the tested date of 42
• The age of the concrete should be specified when laboratory-produced
curing days, the report says. The 365-
mixtures are being evaluated and correlated to long-term ponding tests.
day, long-term ponding tests also
• All laboratory-produced specimens for coulomb and ponding testing should
showed that heat-cured slabs generally
be moist-cured for no more than seven days. Moist curing to ages or 28 or
had the lowest near-surface chloride
90 days should be prohibited as unrealistic.
concentrations compared to the average
• Long-term ponding tests should be specified to mean at least 365 days.
of the other types.
• Salt-water solutions in long-term ponding tests should be increased to 15
The silica-fume mixtures produced
percent salt from the current three percent.
28- and 180-day compressive strengths
• The existing tables showing correlations between coulomb ratings and
equal to the strengths of conventional
permeability should be eliminated totally.
concretes with equal w/c ratio, either
burlap- or tank-cured, the study reports.

7. way to improve current concrete is to
tighten specifications to require the use
of lower w/c concretes. Concrete should
be specified to have a low chloride-
diffusion coefficient as determined by
long-term ponding tests.” Use of the
ASTM standards that provide only
coulomb values should not be used to
specify concrete mixtures because they
do not apply uniformly across all
variables that can be used to reach the
same durability, including different
‘The best way to
improve current
concrete is to tighten
specifications to require
the use of lower w/c
concretes.’
aggregates, admixtures, and curing
procedures. “The tests must be used
only when proper correlations between
coulomb values and long-term ponding
test results have been established for the
individual concretes under test, as
already required in the 1991 ASTM C
1202.”
The study suggests that the precast
industry should carry out long-term
ponding tests, of at least one year in
duration, to prove the performance of
ASCENT, SUMMER 1996
the material to all purchasers. “Taking a
long-term view of concrete performance
can increase the durability of all
structures, lessening the need for repair
and maintaining concrete as a state-of-
the-art building material of choice for
34 the transportation and construction
industry.”
These two studies provide owners
and designers with more insight into
how to create their precast concrete
structures—and how to avoid costly
KEY mistakes. Everyone benefits when those
involved in design, construction, and
maintenance are aware of the material’s
design properties and how long-term
care combine to keep costs to a
These three diagrams fromWJE’s chloride-permeability study compare permeability levels of minimum. ■
various curing methods for concretes with water/cement ratios of 0.46 (top), 0.37 (middle) and
0.32 (bottom). They show that no matter the curing method used, durability improved as the
w/c ratio was lowered. — Craig A. Shutt
lower than that assumed in other lower than the surface concentrations
published corrosion research for long- for the overall average of the burlap-
term life estimates.” The heat-cured and water tank-cured concretes with
concrete surface concentrations were and without silica fume, at all w/c levels.
between 18 percent and 36 percent Overall, the study states, “The best